Internal combustion engines frequently employ boosting devices to compress the airflow before the air enters the intake manifold of the engine in order to increase power and efficiency. A common boosting device employed in internal combustion engines is a turbocharger that uses a gas turbine motivated by the engine exhaust flow to drive a gas compressor for engine intake air. Some engines employ variable-geometry turbochargers (VGTs). A VGT is a type of a turbocharger usually designed to allow the effective aspect ratio (A:R) of the turbocharger to be altered in line with engine speed, and thus facilitate increased engine operating efficiency. VGTs tend to be more common on compression-ignition or diesel engines, as compared to spark-ignition or gasoline engines, because lower exhaust temperatures of a diesel engine provides a less extreme environment for the movable components of the VGT.
Exhaust braking is a means of slowing a diesel engine by closing off a path of exhaust flow from the engine when fuel to the engine has been shut off. Such closing off the engine exhaust flow generates backpressure inside the engine by causing the exhaust gases to be compressed inside the engine's exhaust manifold and its cylinder(s). Since the exhaust gases are being compressed and there is no fuel being supplied, the engine rotation is impeded, thus slowing down the vehicle. Exhaust braking essentially creates a major restriction in the exhaust system, and creates substantial exhaust backpressure to retard engine speed and offer some supplemental vehicle braking In most cases, an exhaust brake is so effective that it can slow a heavily loaded vehicle on a downgrade without ever applying the vehicle's service, such as friction, brakes.